Astronauts pee using a specially designed funnel and hose that pulls urine away from the body with airflow, replacing the gravity that normally does the job on Earth. The system sounds simple, but the engineering behind it is surprisingly complex, and most of that urine eventually becomes drinking water.
How the Space Toilet Actually Works
The current system on the International Space Station is called the Universal Waste Management System (UWMS). To urinate, astronauts hold a specially shaped funnel connected to a hose up to their body. A dual fan separator creates a steady stream of airflow through the funnel, pulling urine into the hose the way a vacuum pulls in dust. Without that airflow, urine would simply float away as a blob of liquid.
Once inside the system, the urine enters a spinning separator that uses centrifugal force to split the liquid from the air. The urine gets routed to storage containers, while the air passes through a carbon-based filter to trap odors before being released back into the cabin. The funnel and hose handle urine collection separately from the seat used for bowel movements, though the two can be used at the same time. That simultaneous design came directly from feedback by female astronauts, whose anatomy makes it impractical to use one without the other.
What Happened Before Modern Space Toilets
Early space missions had far cruder solutions. During the Apollo program, astronauts attached roll-on cuffs to themselves, essentially condom-like sleeves connected to a collection bag and transfer tube. Urine flowed through the tube into a tank, and most of it was simply vented into the vacuum of space. A small portion was freeze-dried and brought back to Earth for medical testing. Astronauts from that era have described the vented urine crystallizing instantly into a glittering cloud of ice particles outside the spacecraft, one of the more unexpectedly beautiful sights in orbit.
The Gemini and Mercury programs were even more primitive, relying on bags that astronauts had to manage by hand in extremely tight quarters. The shift to suction-based systems on the Space Shuttle in the 1980s was a major quality-of-life improvement, and the UWMS installed on the ISS in 2020 refined the concept further with a smaller footprint, better airflow, and improved comfort.
Your Pee Becomes Someone Else’s Coffee
Launching water into orbit costs roughly $10,000 per pound, so recycling every possible drop is essential. The ISS Water Recovery System processes urine, sweat, and even moisture from exhaled breath into clean drinking water. The first stage, the Urine Processor Assembly, heats pretreated urine in a low-pressure distillation system and recovers about 85 to 87% of the water content.
The leftover concentrate, called brine, used to be simply stored and disposed of. But a newer technology demonstration called the Brine Processor Assembly now squeezes additional water out of that brine, pushing total water recovery from urine up to about 98%. Before the brine processor was installed, overall water recovery on the station hovered between 93 and 94%. That jump to 98% is a significant milestone for planning missions to Mars, where resupply from Earth won’t be practical.
The remaining 2% of brine that can’t be converted to water gets packed into cargo vehicles that burn up on reentry. NASA is studying whether that leftover brine could serve as fertilizer for plants grown in space, since it still contains nitrogen and phosphorus. For now, though, it’s treated as waste.
Why Peeing in Space Is a Health Concern
The act of urinating in microgravity isn’t just an engineering problem. It’s also a medical one. Without gravity constantly pulling on the skeleton, astronauts lose bone density over the course of a mission. That lost calcium enters the bloodstream and gets filtered out through the kidneys, significantly raising calcium levels in the urine. Higher urinary calcium means a higher risk of kidney stones, which would be a serious medical emergency hundreds of miles from the nearest hospital.
NASA monitors astronauts’ urinary calcium levels throughout a mission as a real-time indicator of how much bone loss is occurring. Both body weight and time spent in space correlate with higher calcium excretion, meaning the risk increases the longer a mission lasts. Exercise regimens on the ISS, particularly resistance training, help slow bone loss, but they don’t eliminate it entirely. For future Mars missions lasting two to three years, managing kidney stone risk will be an even bigger challenge.
The Practical Experience
Astronauts train extensively on using the toilet before launch, because getting a good seal with the funnel matters. A poor seal means urine escapes into the cabin as floating droplets, which is both a hygiene issue and an equipment hazard. The airflow provides a clear directional cue, so astronauts can feel the suction confirming the funnel is positioned correctly.
The whole process takes about the same amount of time as it would on Earth. The bigger adjustment is the lack of a natural “urge” cue. On Earth, gravity pulls urine to the bottom of the bladder, triggering the feeling that you need to go. In microgravity, urine distributes more evenly inside the bladder, so astronauts often don’t feel the urge until their bladder is much fuller than usual. Many report needing to consciously remind themselves to use the toilet on a regular schedule to avoid discomfort or potential bladder issues.